In the process of establishing an oil or gas well, the well is typically provided with an arrangement for selectively excluding fluid communication with certain zones in the formation to avoid communication with undesirable fluids. A typical method of controlling the zones with which the well is in fluid communication is by running well casing down into the well and then sealing the annulus between the exterior of the casing and the walls of the wellbore with cement. Thereafter, the well casing and cement may be perforated at preselected locations by a perforating gun or the like to establish fluid communication with product bearing zones in the formation. The cement also prevents the fluids in adjacent zones which are otherwise sealed from the zone of interest by a shale, a fault, or other geological condition from bypassing the geological seal by moving along the wellbore or well casing. Unfortunately, places where the casing is in contact with the walls of the wellbore do not get surrounded by cement and do not seal the wellbore from migrating fluids.
A number of devices, which are typically called centralizers, have been developed to space the pipe string from the walls of the wellbore during the cementing process. An example of a typical centralizer is a bow spring centralizer which comprises a plurality of elongate spring metal strips which bow outwardly from the pipe string. The bow springs are typically provided at the collars of the well casing in sets to push the casing away from the walls of the wellbore. However, during installation of the string into the wellbore the bow springs create substantial frictional forces reducing the potential reach of a well. Also, the bow springs are somewhat fragile and subject to failure.
Another example of a centralizer for cementing operations is U.S. Pat. No. 2,654,435 issued on Oct. 6, 1953 to Oliver. The Oliver device comprises a shoe attached to the end of the casing string wherein the shoe includes bow springs which are held in a collapsed position by a stem extending through the wall of the shoe to an interior retainer. When the string is in the desired position in the wellbore, the casing string is pressurized to force a plug from an aperture in the end of the shoe. The plug is connected to the retainer which releases from the stem when the plug is forced from the aperture which releases the bow springs to centralize the casing. As an alternative arrangement two or more shoes could be installed in the same string with the retainers connected along a shaft to the end plug. Clearly, this system comprises a complicated deployment apparatus which may be subject to failure or premature deployment. Moreover, it would be impractical for a large number of centralizing shoes to be installed in a casing string which may be necessary in a horizontal well since it must rely on the one plug.
Accordingly, it is an object of the present invention to provide a method and apparatus for centralizing pipes in a wellbore which overcomes or avoids the above noted limitations and disadvantages of the prior art.
It is a further object of the present invention to provide a method and apparatus for spacing a pipe from the walls of a wellbore which remains within the profile of the pipe while the pipe is moved into and around the wellbore.
It is an additional object of the present invention to provide a method and apparatus for spacing a pipe from the walls of a wellbore wherein the mechanism for spacing may subsequently be used for fluid communication between the pipe and the wellbore.